Led 12v project

[Swaynie]

Hi im new to this forum and a novice when it comes to led's resistors etc.

Im trying to make a simple cost effective led rear light system for my car, The issue i have is i cant just wire up some led's and work as my car produces 14v ad the leds would last 2 mins and blow.

So i need a system that uses resistors to keep the volts at a constant 12v to avoid lowing led's.

this is a kit supplied in italy and is over £300 which for the cost of led's and parts is OTT and the system only uses outer leds and the center lights are ordinary bulbs.

Im after making a complete led light set which hasnt been done yet, If i can get the basics of were to put the resistors or what ever i need to stop my led's burning out would be muchly appreciated.

P.s im pretty handy with a slodering iron

Any help would be great

Thankyou

 

[MrDEB]

first you need to know the specs of the LEDs then caculate what resistor you need. as a suggestion, don't go to the maxium current as the voltage changes in and automobile. You really can't tell much differene running the LEDs at 20ma vers 15ma.
LED Resistor Calculator
Multiple LEDs are best n series, not parallel

 

[Swaynie]

ok thanks i did the test on thats site and i cant really tell the results as it shows a -

Oh and im thinking of using 15000mcd brightness aswel but im unsure if these will be too bright or not bright enough

 

[k7elp60]

You have to add up the individual LED voltages, you cannot run 10ea 1.8V LED's on 14.5 volts. Even 6 in series may be pushing it, as with the engine not running the battery voltage my drop to 12.8 or so. The 1.8V is for red LED's, if you are using white LED's the voltage is about 3.0 volts. What you do is create multiple series strings that are connected in parallel. Each string has a resistor.

 

[vne147]

The problem is that you entered into the calculator that you want to place 10 LEDs in series. Each LED has a voltage drop of 1.8 V so 10 of them in series would have a total voltage drop of 18 V. Unfortunately, your supply is only 14.5V which as you can see is not enough. The caluclator is coming up with a negative volatge that must be dropped by the resistor and therefore spits out a negative resistance. You need to either decrease the number of LEDs in series so that the total voltage drop is less than the supply voltage or place some LEDs in series and some in parallel. For example you could have 2 banks of 5 LEDs each wired in parallel.

 

[Swaynie]

so running a complete parallel setup will gain as shown below and then there will be less strain on the cars alternater which pumps out a constant 14.5-8 volts and 100amps

 

[vne147]

You can wire all LEDs in parallel if you want but I would do 2 banks of 5.

If you wire them in parallel you would need a resistor to drop the remaining voltage after the LED. That's 14.5V (supply voltage) - 1.8V (LED forward voltage) = 12.7V.

If you multiply that 12.7V by the current going through the LED and resistor (.02A) you get the power dissipated by the resistor.

P = IV

12.7*(.02) = .254 W

Multiply that by 10 (the number of LEDs) and you get .254*10 = 2.54W.

That's 2.54 watts that you're just wasting and in full parallel arrangement your electrical system must supply .02 * 10 = 200 mA.

If you had two banks of 5:

14.5 - 5*1.8 = 5.5V

5.5*.02 = .11 W

Multiply that by 2 for 2 banks of 5 and you get .22W that just gets wasted and your electrical system only has to supply .02*2 = 40 mA of current.

The parallel/series combination definetly puts less "strain" on your electrical system.

 

[Swaynie]

so in a circle if anyone is good at drawing circuits how would these look?

 

[vne147]

Like this:

 

[audioguru]

LEDs each have a different voltage so they cannot be connected directly in parallel. Make series strings of LEDs that add up to about 80% of your minimum supply voltage then calculate a series current-limiting resistor using simple arithmatic, not a stupid Wizard.

If the LEDs are 1.8V and the minimum supply is 12.0V then use 5 LEDs in series which add to 9V. The 12V is minimum and 13.6V is average so the resistor is (13.6V - 9V)/20mA= 230 ohms. Use 240 ohms which is the nearest standard value. When the alternator is running fast the voltage is 14.4V then the current is (14.4V - 9V)/240 ohms= 22.5mA which is fine. The 240 ohm resistor dissipates (22.5mA squared x 240 ohms) = 122mW so use a 1/4W resistor.

Cheap bright LEDs use a focussed case that makes them appear bright but only at an angle that is too narrow for a vehicle. You need LEDs that have a fairly wide angle and are fairly bright.